15 



the start, and by proper care endeavors to keep them in this condition until the 

 product is matured, he will have accomplished more in preventing loss from fungous 

 diseases than he would accomplish by the best remedies known applied to half- 

 starved plants." 



We know also that certain varieties of a species of plant are more resistant 

 than other varieties of the same species under similar treatment. We are beginning 

 to understand some of these cases through the recent studies of the laws of heredity 

 as revealed in JVIendelism. In some cases it has been shown that immunity and 

 susceptibility are hereditary, transmissible characters. It is possible, therefore, to 

 combine in a single'plant through hybridization immunity with other good qualities. 



Without discussing the great importance of this phase of plant breeding, I 

 shall pass on to discuss another phase which has been recently studied, viz., some 

 factors relating to the immunity of plants to disease. 



IMMUNITY OF PLANTS TO DISEASE 



It has long been a matter of common observation that some plants are less 

 susceptible, or are more immune, to disease than others. It has frequently been 

 noted, for example, that individual plants in a field, garden, or orchard escape the 

 ravages of some disease which overtakes the others. As to the cause of this im- 

 munity, botanists are not agreed. For a long time the main factor in immunity 

 was considered to be the structure of the external coat or epidermis. For example, it 

 was believed that plants with a thick epidermis, or with few stomata, or with a 

 thick coating of hairs, were more resistant to disease than plants with a thin 

 epidermis, or with many stomata, or with few hairs. Recent researches, however, 

 go to show that the anatomical structure has nothing to do with the question of 

 immunity. Immunity depends upon the physiological reaction of the fungous 

 protoplasm to the cells of the plant attacked. The cells of the susceptible host 

 plant contain chemical substances which have the power of directing the move- 

 ments of small organic cells, either positively by attacking them, or negatively by 

 repelling them. 



It has been observed that in some cases the fungous spore germinates and 

 enters the stomata, but is unable to develop a mycelium within the tissues. This 

 result is probably due to the formation of anti-toxins, or other substances, which 

 destroy the enzymes or toxins of the fungus. 



Infection occurs, therefore, when the anti-body in the host is unable to over- 

 come the enzyme or toxin of the fungus, and immunity is the condition of the host 

 when the anti-body is able to destroy the toxin or enzyme of the fungus. 



